refactor: remove dead code, hollow stubs, and orphaned modules

Backend — delete 6 orphaned schema sub-modules never imported by any route:
  compliance/schemas/{ai_system,bsi,dashboard,isms_governance,report,service_module}.py
  All symbols were only accessible through the monolithic schemas.py barrel;
  the split files were never wired up and created misleading import paths.

Frontend — delete 2 hollow "future implementation" stub pages:
  app/sdk/agents/sessions/page.tsx
  app/sdk/agents/statistics/page.tsx
  Both showed a static placeholder with no API calls or real functionality.

Frontend — delete dead demo seed/clear API routes:
  app/api/sdk/v1/demo/{seed,clear}/route.ts
  Used a global in-memory store never connected to PostgreSQL.
  The actual demo seeding (lib/sdk/demo-data/) goes through the real
  state API (apiClient.saveState/deleteState), not these routes.

Frontend — remove unimplemented PDF export alert from retention page:
  app/sdk/einwilligungen/retention/page.tsx
  Removed the PDF button and alert('PDF-Export wird noch implementiert.')
  stub. CSV and JSON export remain fully functional.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

admin-compliance/app/sdk/einwilligungen/retention/page.tsx

@@ -274,7 +274,7 @@ function RetentionTimeline({ dataPoints, language }: RetentionTimelineProps) {
 // =============================================================================
 
 interface ExportOptionsProps {
-  onExport: (format: 'csv' | 'json' | 'pdf') => void
+  onExport: (format: 'csv' | 'json') => void
 }
 
 function ExportOptions({ onExport }: ExportOptionsProps) {
@@ -294,13 +294,6 @@ function ExportOptions({ onExport }: ExportOptionsProps) {
         <Download className="w-4 h-4" />
         JSON
       </button>
-      <button
-        onClick={() => onExport('pdf')}
-        className="flex items-center gap-2 px-3 py-2 bg-indigo-600 text-white rounded-lg text-sm hover:bg-indigo-700"
-      >
-        <Download className="w-4 h-4" />
-        PDF
-      </button>
     </div>
   )
 }
@@ -332,7 +325,7 @@ function RetentionContent() {
   }, [allDataPoints, filterCategory])
 
   // Handle export
-  const handleExport = (format: 'csv' | 'json' | 'pdf') => {
+  const handleExport = (format: 'csv' | 'json') => {
     if (format === 'csv') {
       const headers = ['Code', 'Name', 'Kategorie', 'Loeschfrist', 'Rechtsgrundlage']
       const rows = allDataPoints.map((dp) => [
@@ -354,8 +347,6 @@ function RetentionContent() {
         legalBasis: dp.legalBasis,
       }))
       downloadFile(JSON.stringify(data, null, 2), 'loeschfristen.json', 'application/json')
-    } else {
-      alert('PDF-Export wird noch implementiert.')
     }
   }